i would have to say either my Legion Frumentarii or my Regulator. Both seem to have the most depth when it comes to what i can do in-character in the game and i greatly enjoy using both. especially since they are very underpowered (and i do play them on hardcoe) as the Frumentarii wears Merc Cruiser gear and the Regulator wears a Regulator Duster so they are more challenging to survive as
I played a Legion Frumentarii, and really enjoyed that... until it arrived at a point where I didn't like the character on a personal level. He was really not a very nice guy. While I don't mind playing evil, I have to be able to find something I like in the player, or the joy is gone. One of my favorites was a "Happy hoker" who opened the "Goodsprings Social Club" in the old abandoned shack near Goodsprings.
She was fun because she was a liar, thief, and sometimes assassin who was basically a helpful character to those she encountered... for a price...